Myron G. Best. Igneous and metamorphic 2003 Blackwell Science

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Chemical Zoning. Larger grains of solid solutions,

especially porphyroblasts, in metamorphic rocks are

commonly chemically inhomogeneous. Compositional

zoning more or less concentric with the grain margin

can develop by fractional crystallization; the interior

of the growing porphyroblast was isolated from the

surrounding reactant phases in the matrix because

changes in metamorphic conditions proceeded faster

than compositional equilibration by ionic diffusion

(Section 16.5.2). During growth as a product phase

from a continuous mineral reaction, there was a con-

tinuous change in the composition of the material

supplied to the surface of the enlarging porphyroblast.

For zoning to be preserved after growth, the rate of

diffusion of unevenly concentrated ions must be very

slow, as otherwise the grain would have at least par-

tially homogenized and equilibrated with its surround-

ings. If no retrograde equilibration has taken place,

the rim composition is part of the mineral assemblage

of neighboring matrix phases produced under peak

metamorphic conditions. Internal compositions and

any inclusions can be used to infer earlier metamor-

phic conditions (Figure 16.35). In some cases, parts of

zoned minerals can be dated, allowing construction of

P–T–t paths.

Porphyroblasts of garnet are widespread and com-

monly zoned. Although the zoning is not optically

visible in thin section, spot analyses using an electron

microprobe reveal the nature of zoning (Figure 16.34).

Back-scatter electron images can also show the zon-

ing, provided it involves a change in average atomic

number (e.g. changes in Fe versus Mg).

Many zoned almandine-rich garnets in pelites are

produced by the continuous prograde reaction chlorite

 quartz  garnet  water. Because Mn-rich garnets

are stable at lower temperatures than more Fe–Mg-rich

ones, the cores of garnets typically have much higher

Mn, or spessartine end-member, concentrations.

Chernoff and Carlson (1999) have found that some

Fe–Mg–Mn garnets apparently achieved chemical

equilibrium with their surroundings during diffusion-

controlled growth but Ca and trace elements in them

did not because of slow rates of intergranular diffusion.

Caution needs to be exercised in using thermobaro-

meters based on Ca in garnet.

Interpretation of concentration maps and zoning

proﬁles can be challenging, given the complexities of

Evolution of Imposed Metamorphic Fabrics: Processes and Kinetics

527

(b)

(a)

70 80

0.2 mm

135

0.2 mm

138

Quartz

Muscovite

0 0.5mm

17.6 Compromised grain boundary conﬁgurations in aggregates

of amphibole and biotite. (a) Triple junctions depart from

120° angles because of the inﬂuence of the surface-energy

anisotropy of the adjoined monoclinic amphibole grains.

(b) Corrugated or sawtooth boundary between two biotite

ﬂakes with oblique (001) cleavages. Compared to a smooth

boundary of lesser surface area, the sawtooth one has larger

area but smaller overall energy and is more stable. Redrawn

from Kretz (1966).

17.7 Stable texture in a quartz–muscovite schist under cross-

polarized light. Euhedral ﬂakes of mica are interspersed

among unusually shaped, rectangular quartz grains. In a

monomineralic quartzite, grain equilibrated shapes would be

relatively equant polygons meeting at 120° triple junctions.

some growth patterns (Figure 17.2), in some instances

compounded by concurrent bodily rotation of the

entire porphyroblasts (see Figure 17.30), overgrowths

on deformed matrix foliation (see Figure 17.32), and

the cut effect (discussed later).

Inclusions. Most porphyroblasts contain inclusions of

one or more other crystalline phases. Although these

poikiloblasts are unstable as a result of the internal

surface area relative to an inclusion-free porphyroblast

of equivalent composition and size, the inclusions are

effectively armoured against reactive communication

with matrix phases because of sluggish kinetics and,

hence, can persist indeﬁnitely. Poikiloblasts potentially

provide further insights into processes and events dur-

ing their growth in addition to any information from

compositional zoning. Inclusions are petrogenetically

useful for the following reasons:

1. Some are preserved reactant phases and provide in-

formation on the nature of mineral reactions that

created the poikiloblast. For example, the ﬁrst

appearance of andalusite or kyanite (depending on

P) in prograding pelites might be related to the

discontinuous reaction decomposing pyrophyllite

17.1 Al

(OH)

 Al

SiO

 3SiO

 H

The three moles of quartz produced for every one

mole of Al

SiO

cannot readily be removed by dif-

fusion of Si nor physically “bulldozed” in front of

the advancing crystal-growth front; consequently,

byproduct quartz remains as armoured inclusions

within the andalusite or kyanite poikiloblast. Some

or all of the reactant phases involved in growth of

the poikiloblast may have been consumed by sub-

sequent reactions in the surrounding matrix but

their preservation within the poikiloblast testiﬁes of

their prior existence. Other inclusions, such as zir-

cons, may never have participated in poikiloblast-

forming reactions and were simply physically

engulfed as an “inert” phase into the growing poik-

iloblast. Inert graphite particles are adsorbed along

preferred crystallographic domains in growing

andalusite (Figure 14.10).

2. Some inclusions can be used as geothermobarome-

ters. An example was described in Section 16.11.2

of zoned garnet poikiloblasts in the Wopmay

orogen; the presence of kyanite inclusions in the

interior and sillimanite in the outer parts of the

poikiloblasts supports a decompression path dur-

ing growth determined by thermobarometry.

3. Some inclusions can be dated isotopically, furnish-

ing an absolute chronology for construction of

P–T–t paths.

4. Insights can be gained from trails of inclusions

about the history of deformation of the rock, either

before or during poikiloblast growth. This is de-

scribed in Section 17.3.3.

Exsolution Textures and Symplectites. As intensive

variables change, destabilized, initially homogeneous

minerals can experience breakdown into two or more

crystalline phases. Careful attention to textures and

awareness of critical phase equilibria may be necess-

ary to distinguish these exsolution intergrowths from

superﬁcially similar inclusions in poikiloblasts.

The most widespread exsolution texture is that seen

in alkali feldspar perthites. Though previously dis-

cussed as forming in slowly cooled magmatic granitic

systems (Figures 5.17 and 5.18a), perthites are also

common in slowly cooled high-grade metamorphic

rocks. Some pyroxenes in high-grade rocks also display

exsolution into Ca-rich and Ca-poor intergrowths.

Symplectites, or symplectitic intergrowths, are gen-

erally two-phase vermicular (wormlike) breakdown

products of high P–T solid solutions. Common ones

include cordierite  quartz  orthopyroxene after

garnet and plagioclase  diopside replacing Na–Al-

rich omphacitic pyroxene (Figure 15.11) in maﬁc

eclogite-facies rocks. Other symplectites result from

mutual reaction between two adjacent unstable phases,

such as orthopyroxene  spinel  clinopyroxene at the

junction between olivine and plagioclase (Figure 16.6).

Symplectites result from diffusion-controlled reactions

where product phases grow as close as possible to

reaction sites, or to the reaction sites liberating the

slowest diffusing component.

17.2 DUCTILE FLOW

Anisotropic tectonite fabric is characteristic of broad

regional terranes in orogenic belts that constitute the

most widely exposed metamorphic rock on Earth.

Under the elevated pressures and temperatures that

prevail in the deep crustal parts of these orogens and

in concert with generally slow rates of strain under

nonhydrostatic states of stress, rocks deform by grain-

scale, pervasive ductile ﬂow. As a result, metamorphic

bodies change shape, or strain, without loss of cohe-

sion. Cracking, or fracturing, and subsequent frictional

sliding can function locally in metamorphic rocks at

high strain rates and where ﬂuids reduce the effect-

ive stress, but these brittle mechanisms are typically

focused along shallow crustal faults at lower pressures

and temperatures than those prevailing in metamor-

phic terranes. Broadly concurrent recrystallization and

ductile deformation is responsible for the characteristic

anisotropic tectonite fabric of the class of rocks called

tectonites that typify regional metamorphic terranes in

orogens of all ages.

Ductile ﬂow is a standard topic of structural geo-

logy textbooks, such as Twiss and Moores (1992; see

528 Igneous and Metamorphic Petrology

also Sections 8.1 and 8.2.1 and Snoke et al., 1998,

pp. 9–15). However, the “gray area” between structural

geology and metamorphic petrology—or structural

petrology—provides rich insights into the develop-

ment of tectonite fabrics and is accordingly pursued

here. Historically, most of our initial understanding

of ductile ﬂow came from metallurgical practices and

research because shaping pieces of metal by bending,

extruding, and rolling involves ductile ﬂow. But, dur-

ing the decades of the late twentieth century, experi-

mental studies on the ductile behavior of rocks and

rock-forming minerals ﬂourished (Figure 17.8), as did

careful observations from the scale of an outcrop down

to that seen by an electron microscope.

Large-scale ductile deformation of rock bodies is

accomplished by the integrated effects of two basic

grain-scale mechanisms that act in a pervasive manner

throughout a rock volume: ﬁrst, a chemical transfer

called diffusive creep and, second, intracrystalline

plasticity and related recrystallization called dislocation

slip and creep. Both mechanisms are stress-induced

and are thermally activated; that is, promoted at higher

T. They result in grain strain, but do not modify the

crystalline nature of the grains. Each ductile mechan-

ism has associated kinetic rates, an essentially distinct

associated fabric, and a speciﬁc range of conditions

over which it is the dominant mechanism of ductile

ﬂow (Figure 17.9). A third process—called grain-

boundary sliding—involves rotational and translation

motion between neighboring grains to accommodate

gaps and overlaps in aggregates as they deform by the

other two mechanisms.

17.2.1 Diffusive Creep

Mineral grains in nonhydrostatically stressed rocks can

change shape by thermally activated diffusive transfer

Evolution of Imposed Metamorphic Fabrics: Processes and Kinetics

529

2.5 mm .25 mm 0.1 mm

17.8 Solid-state ductile ﬂow in marble produced in the laboratory. A small cylinder of marble at 500°C subjected to a conﬁning pressure of

5 kbar was extruded into a hole in the loading piston (Figure 8.5). The rate of strain in the extruded region was 3  10

7

/s, or the

approximately 100% strain was accomplished in about 35 days. (a) Photomicrograph shows a thin section of the deformed cylinder un-

der cross-polarized light. Extruded neck broke after termination of experiment. (b) Enlarged view of extruded region showing slender

deformation twin lamellae in the calcite grains, especially prominent in and near the extruded region, and region of extensive recrystal-

lization into polygonal grains above them. (c) Enlarged view of granoblastic grain aggregate produced by recrystallization that replaced

intensely strained grains; one slender twin lamella remains. Photographs and information courtesy of Hugh C. Heard. From Heard HC.

1963. Effect of large changes in strain rate in the experimental deformation of Yule marble. J. Geol. 71: 162–195. Reproduced with permis-

STRESS

Dislocation slip

Dislocation

creep

and

Diffusive

transfer

INCREASING

STRAIN RATE

17.9 Highly generalized deformation map showing the relative

range of conditions over which important mechanisms of duc-

tile deformation occur. Only mechanisms allowing potentially

large amounts of strain by steady-state creep (slow continuous

deformation) are represented. See Twiss and Moores (1992,

p. 387) for more speciﬁc details.

of atoms from one part of a grain to another. The

driving force for this diffusion is a gradient in chemical

potential from high- to low-stress sites (Green, 1980).

In general, grains shorten parallel to the maximum

compressive stress axis and lengthen parallel to the

minimum compressive stress (Section 8.1.1). The ﬂat-

tened grains impart a foliation to metamorphic rocks.

Deformation by diffusive transfer has a rheologic

behavior resembling that of Newtonian viscosity where

the time rate of strain is proportional to the applied

shear stress (Section 8.1.3). Like viscous ﬂow, diffusive

transfer is a slow continuous deformation that occurs

under steady shear stress at low strain rates, hence the

appellation diffusive creep.

Three possible creep processes correspond to the

three different diffusion paths in grain aggregates (Fig-

ure 17.1) that have contrasting rates. Diffusion through

the volume of the grain is Nabarro–Herring creep, dif-

fusion along dry grain boundaries is Coble creep, and

via an intergranular ﬂuid is pressure solution.

All three diffusion paths are effective at relatively

high temperatures but other thermally activated mech-

anisms of ductile ﬂow are likewise enhanced and may

supersede the effects of diffusive transfer and creep.

Hence, diffusive creep is generally manifest at relatively

low temperatures of metamorphism, limited perhaps

to the upper greenschist facies. In generally coarser

grained, higher grade metamorphic rocks, paths of dif-

fusion are lengthened and there is proportionately less

surface area to volume; both factors decrease the effect-

iveness of diffusive transfer. In addition, higher grade

rocks may have little ﬂuid in them as a result of pro-

grade devolatilization reactions. Locally, however, as in

brittle fault and ductile shear zones, grain size can be

reduced by cataclasis (Section 8.2.1 and Figure 8.7)

and by dynamic recrystallization (discussed later),

respectively. In these sheets of deformed rock, strain

can be enhanced by diffusive creep.

Pressure Solution. Probably the most effective mech-

anism of diffusive creep is pressure solution because

of the enhanced kinetics where intergranular ﬂuids are

present.

The work of stressing a crystal imparts increasing

energy as strain is locked in; equilibrium atomic bond

angles and distances are changed. In the presence of a

ﬂuid, a stressed, higher-energy crystal is more soluble

relative to neighboring ones in the communicating

ﬂuid that are not stressed and the surface of a single

grain where the normal stress is greatest is more

soluble than where the stress is less. As a result of this

Riecke’s principle, higher concentrations of dissolved

chemical species are created in intergranular ﬂuids

adjacent to more stressed grains or grain parts. The

resulting concentration (activity) gradient drives ionic

diffusion from more stressed grain margins to less

stressed surfaces where grain growth takes place. In a

nonhydrostatically stressed rock permeated by an inter-

granular ﬂuid, this differential pressure solution can

produce a signiﬁcant change in shape of the rock body.

Pressure solution depends on the availability of relat-

ively soluble minerals under metamorphic conditions,

the most common of which are calcite and quartz. The

necessary ﬂuid can be derived from different sources

(Section 16.6.1), but ﬂuid-liberating compaction and

prograde reactions are probably signiﬁcant.

Pressure solution can be manifest in different ways

in rocks. It locally creates a signiﬁcant loss in rock

volume. In some conglomerates, depressions on the

surfaces of clasts are created by the pressure of neigh-

boring impinging clasts causing pressure solution. In

ﬁne-grained sedimentary and low-grade metamorphic

rocks, dissolution surfaces are repetitively spaced

surfaces millimeters to centimeters apart that deﬁne a

spaced cleavage (Section 15.1.1; see also Twiss and

Moores, 1992, Chapter 13). Some of these dissolution

surfaces give a false impression of differential shear

movement parallel to the surface (Figure 15.4).

Irregular stylolites in carbonate rocks where fossils are

sharply truncated are examples. In low-grade meta-

morphic rocks, less soluble phyllosilicates and other

“inert” minerals, such as zircon, can be concentrated

along discrete crenulation cleavages where relatively

more soluble quartz has been preferentially dissolved

and removed (Figure 15.3). Pressure solution can also

be manifest on a pervasive grain scale throughout a

rock, imparting a penetrative foliation. Thus, ﬂattened

quartz grains in some schists and phyllites have a ﬁlm

of insoluble residue on the ﬂat margins and ﬁbrous

overgrowths (“beards”) on the extended margins (Fig-

ure 17.10). These overgrowths resemble mineral pre-

cipitates in the lower stress pressure shadows alongside

rigid objects such as boudins (Figure 15.8b).

The existence of interconnected ﬂuid pathways

along grain boundaries is essential for pressure solu-

tion. Under hydrostatic states of stress, H–O–C ﬂuids

in aggregates occur in isolated pools at grain corners

where the dihedral angle  60° and possibly in tubes

along three grain edges where the dihedral angle 0° 

60°. However, Tullis et al. (1996) have demon-

strated experimentally that in feldspar aggregates un-

der nonhydrostatic stress at elevated P and T most grain

boundaries are fully wetted, or the ﬂuid is a continuous

ﬁlm covering the entire grain boundary; the dihedral

angle is effectively equal to 0°. This deformation-

induced ﬂuid wetting of grain surfaces reduces the

yield strength of the aggregate while increasing the

diffusive transport rate, both by about an order of mag-

nitude. Diffusive creep is promoted at the expense of

plastic dislocation creep, which otherwise prevails at the

P and T of the experimental conditions. Deformation-

enhanced pressure solution accomplishing diffusive

530 Igneous and Metamorphic Petrology

Original grain

(a)

Grain strained by

intracrystalline

plastic slip

Grain strained by

pressure solution

(b)

0 0.05mm

(c)

0 0.05mm

17.10 Flattened quartz grains resulting from pressure solution. (a) Schematic comparison of intracrystalline plastic slip and pressure solution,

where C is the corroded dissolution surface resulting from removal of material and O is a segment of the original grain surface marked

by minute insoluble impurities onto which new grain material has been deposited as an overgrowth. The volume of the grain remains

essentially unchanged as material moves from C to O. (b) and (c) Thin sections showing an initially more equant quartz grain that has

been ﬂattened by an overgrowth of optically continuous quartz extending the original grain boundary O into a strain shadow at the

expense of dissolution at the surface C. From Elliott DS. 1973. Diffusion ﬂow laws in metamorphic rocks. Geol. Soc. Am. Bull. 84:

Society of America.

Distorted atomic bonds

due to nonhydrostatic

(shear) stress

ELASTIC STRAIN

Shortened interatomic

distances due to

hydrostatic pressure

Original unstrained

crystal lattice

(a)

(b)

(c)

PLASTIC STRAIN

By crystallographically

controlled

intracrystalline slip

(d)

PLASTIC STRAIN

By twinning in

crystallographically

controlled direction

17.11 Contrast between elastic and plastic strains (Figure 8.3) shown by schematic sections through a hypothetical, two-dimensional crystal lat-

tice. Unstrained lattice is shaded. In the lattice strained by twinning (d), the original lattice points are indicated by dots and the

arrows show they have moved progressively farther the more distant they are from the twin plane (dashed light line).

creep has a potentially important bearing on ﬁne-

grained, mylonitic ductile shear zones in the crust

where strain was apparently focused into weaker rock.

Fluid transport appears to be enhanced in the my-

lonitic sheets relative to surrounding rock, if mobility

of chemical elements (Sinha et al., 1986) and inﬁltra-

tion of meteoric water (Fricke et al., 1992) are any

indication.

17.2.2 Intracrystalline Plastic Deformation

In unstrained crystals, the periodic array of atoms is

maintained in an equilibrium state by interatomic

forces of attraction and repulsion. As stress is applied,

small reversible elastic strains (Section 8.1.1) distort

atomic bonds and interatomic distances are shortened

or lengthened (Figure 17.11a). As the magnitude of a

nonhydrostatic stress increases, permanent bending

and distortion of the atomic structure can be locked

into grains in aggregates. Effects are seen in bent and

otherwise distorted grains, some of which possess un-

dulose or undulatory optical extinction under cross-

polarized light in thin section (Figures 15.17 and

17.12). If P is sufﬁciently great to inhibit brittle rupture

and resulting dilatancy and where the resolved shear

stress is of sufﬁcient magnitude on particular atomic

planes, parts of a crystal yield plastically by slipping

or twinning on those planes into a new equilibrium

position. This intracrystalline plastic strain changes the

external shape of the crystal but preserves the ordering

of the lattice across the slip or twin plane (Figure

17.11c, d). Groups, or domains, of atomic bonds can

be systematically broken or rotated along speciﬁc

crystallographic directions and then reformed into a

new, permanent equilibrium position, while the whole

crystal remains intact and unbroken.

Translation Gliding. In this intracrystalline plastic

mechanism, also simply referred to as slip, discrete

layers of the crystal slide past one another by integral

multiples of fundamental crystallographic units, tem-

porarily breaking and then reforming atomic bonds.

Afterward, the internal lattice is still continuous, cohe-

sion is preserved (the mineral is not cleaved into separ-

ate fragments), and the parts on either side of the slip

panels are still similarly crystallographically oriented;

only the external outline of the crystal has changed.

The ﬂattened overall shape of a mineral grain that

experienced translation gliding, resembling a deck of

slipped playing cards, can be appreciated in Figure

17.11c. The plane on which slip occurs and the direc-

tion of slip in that plane is controlled by the atomic

structure in the crystal. Slip, or glide, planes in crystals

commonly parallel crystallographic directions of dens-

est atomic packing and widest interatomic spacing,

where interplane bonding is weakest. An example is

the (001) plane in micas. These same directions com-

monly parallel crystal growth faces with simple Miller

indices, again as in micas. But the nature of the atoms

in the crystallographic planes is also important, making

for many exceptions.

532 Igneous and Metamorphic Petrology

Plagioclase

Quartz(a) (b)

0 mm 0.5

0mm2

17.12 Slightly deformed granitic rock viewed under cross-polarized light in thin section. Twin lamellae in plagioclase are bent. Most grains,

especially the quartz seen at increased magniﬁcation in (b), have distorted crystal lattices and display undulatory optical extinction

wherein different parts of a single grain extinguish at slightly different orientations as the microscope stage is rotated; extinction sweeps

fanlike across the grain. Strained quartz is no longer uniaxial and can have an optic axial angle as much as 20°.

In halite (Figure 17.13), a prominent slip plane is

{110}, of which, because of symmetry, there are six

planes. The slip direction in one of these planes, (110),

is [110]. Although {111} is the orientation of densest

atomic packing, the alternating layers of Na



and Cl



ions have a strong attraction for one another, so it is not

the easiest slip plane. Crystals of rock-forming minerals

possess one or more slip systems that consist of a slip

plane plus the slip direction on it; speciﬁc ones will be

favored under a particular range of P, T, and strain rate

(Nicolas and Poirier, 1976, Chapter 5). Widespread

quartz in metamorphic rocks slips on basically two

planes—the basal plane (0001) at low T and a prism

plane {1010} at high T.

Quartz grains strained by intracrystalline plastic slip

at relatively low temperatures and/or at high stress or

high strain rate, such as in many mylonites, are severely

ﬂattened and stretched ribbons, inside of which are

thinner, more or less planar deformation lamellae that

mark the active slip planes (Figure 17.14). Comparison

with Figure 17.15 reveals that several hundreds of per-

cent elongation (extension) can be potentially achieved

by constant-volume plastic slip.

Twin Gliding. In this mechanism (Figures 17.11d and

17.16), a portion of the crystal structure responds to a

local shear stress in such a way as to produce a mirror

image of the original crystal. This twin is called a mech-

anical or deformation twin. The twin plane, which is

the mirror across which the parts are symmetrically

related, may be parallel to a plane of plastic slip in

Evolution of Imposed Metamorphic Fabrics: Processes and Kinetics

533

Slip plane

(110)

−

Slip

direction

[110]

17.13 Atomic structure of halite and one of its six symmetrically

related slip planes {110} and directions.

(a) (b)

Deformation

bands

Deformation

lamellae

Margins of

deformation band

0 mm 0.5

0 mm 0.1

17.14 Deformed quartz in a mylonite viewed under cross-polarized light in thin section. (a) Ribbonlike deformation bands of ﬂattened quartz.

(b) Within the bands are oblique deformation lamellae bounded by slip planes. These lamellae have subtle contrasts in refractive index

and can be discerned by very slight adjustments in optical focus in collimated light to emphasize Becke lines. Compare with Figure 17.15.

534 Igneous and Metamorphic Petrology

Original unstrained

grain

Flattening 

 50%

Extension 

 160%

25°

35°

52°

Flattening 

 75%

Extension 

 540%

85°

17.15 Development of a ﬂattened single grain by constant-volume, intracrystalline plastic slip. Two-dimensional model grain is strained by

pure shear as if constricted between the jaws of a vise. (a) Original unstrained grain showing potential slip planes and perpendicular

lattice direction (single line). This grain model could represent the slip system in low T quartz with basal (0001) slip planes and the

crystallographic c-axis as the lattice direction. (b) Flattening strain (Section 8.1.2) 

50% with a corresponding extension (elonga-

tion) of 160%. The crystal lattice has experienced an internal rotation of the crystallographic axis and slip planes by 25° and an external

rotation of the edge of the grain by 52°. (c) Extreme strain and rotation that has created a ribbonlike grain shape by slip on oblique

planes; compare Figure 17.14b. The constraints on this strain model are that the top and bottom faces of the grain remain parallel and

constant in overall orientation and that the dimensions of the lamellae bounded by slip planes remain constant. Redrawn from Hobbs

et al. (1976).

some crystals. Deformation twins may be thin lamellae

and distributed throughout the crystal, as in carbonates

(Figure 17.16c), or they can take over most, and pos-

sibly all, of the crystal, depending upon circumstances.

Twinning seems to be easier in coarser aggregates than

in ﬁner. Wedge-shaped or curved multiple deformation

twins are common in strained plagioclase (Figures

15.17 and 17.12); they contrast with ubiquitous plagio-

clase growth twins in undeformed magmatic rocks that

are straight and parallel and formed while free-ﬂoating

crystals were growing in a silicate melt.

In deformation twinning of calcite, the maximum

achievable shear strain (change in shape) is only 35%

(Figure 17.16). This limitation precludes its inclusion

in Figure 17.9, which refers to ductile mechanisms

allowing large permanent deformation. The geo-

metric aspects of twinning in limestones and low-grade

marbles can be used to infer something of the ori-

entation of the responsible principal stresses and their

magnitudes (Law, 1990).

Kinking and Bending. Differential slip, or twin glid-

ing in some instances, can be restricted to a kink

band in which the slip planes are oriented several

tens of degrees from the same planes in the essentially

undeformed parts of the crystal on both sides (Figure

17.17). Kinking is most commonly found in crystals

that have only one easy slip plane, especially micas, but

also enstatite and kyanite. Kink bands commonly occur

in conjugate pairs more or less symmetrically oriented

to the applied compressive stress directions. Bending

of a crystal lattice at the margins of a kink band or in a

more distributed fashion throughout an entire crystal is

a paradox. Bending requires differential movement

between layers of the lattice, as in bending a pack of

cards. But the bent lattice in Figure 17.18a is inconsist-

ent with plastic slip in Figure 17.11c, where the trans-

lation is only by integral multiples of lattice spacings,

leaving “good” crystal continuously across the plane of

movement. To maintain continuity throughout most of

the bent crystal lattice it is necessary to postulate the

existence of geometrically necessary stacking imperfec-

tions, or defects, shown in Figure 17.18b. These crystal

defects are critically important in plastic strain and are

discussed in some detail below.

Translation Gliding of Crystals in Aggregates. A com-

puter model of deformation of an hypothetical two-

dimensional array of randomly oriented hexagons

possessing one slip plane illustrates what might happen

(a)

(c)

(b)

Evolution of Imposed Metamorphic Fabrics: Processes and Kinetics

535

(a)

Plane

Twin

2

2

(b) (c)

17.16 Twin gliding in calcite on {1102}. (a) Atomic structure across the twin plane showing a small twin (shaded). (b) A small wedge-shaped

twin (shaded) can be produced by compressing the edge of a calcite cleavage rhombohedron with a knife perpendicular to its c-axis.

Twinning occurs at a shear stress of only 100 bar. (c) Lamellar twins (shaded) in a calcite rhomb.

Unstrained,

showing potential

slip planes

Plastically

strained, ends

not constrained

Plastically

strained, ends

constrained

Kink

band

(a)

(b)

0mm1

17.17 Kink bands. (a) Unstrained and plastically strained cylinders of a single crystal with a set of parallel slip planes. The kink band developed

in the third cylinder represents what happens in a crystal that is strained while conﬁned within an aggregate. Extension of a conﬁned

crystal can also produce a kink band. Redrawn from Higgs and Handin (1959). (b) Photomicrograph under cross-polarized light of kink

bands in a plastically strained enstatite crystal in a peridotite.

In aggregates of grains that have random crystallo-

graphic orientation, plastic deformation is obviously

greatly enhanced if the grains are of high-symmetry

and possess multiple slip planes, because of the high

probability that the resolved shear stress is sufﬁcient to

536 Igneous and Metamorphic Petrology

(a)

(b)

17.18 Schematic bent crystal lattice. (a) The arched “card deck”

model does not correspond to bending of a crystal by plastic

slip because of the discontinuity of the lattice across the

surfaces of differential movement. (b) Another model mostly

preserves continuity but embodies geometrically necessary im-

perfections, or edge dislocations, at the ends of half lattice

planes extending part way through the lattice perpendicular to

the plane of the drawing.

(a)

(b)

64°

Undeformed aggregate;

random slip directions

55°

(d)

(e)

Pure shear

 

 35%

49%

l  l

17.19 Computer models of progressive deformation in grain aggre-

gates. (a) Undeformed aggregate. Each grain has one slip

direction and they are randomly oriented in the aggregate. (b)

and (c) Progressive simple shear. Compare Figure 17.20. Note

ruptured grains and overlaps (stippled) and gaps between

grains. (d) and (e) Progressive pure shear. (f) Histograms

showing increasing preferred lattice orientation of slip direc-

tion during progressive strain; dashed lines in histograms for

simple shear are orientations of long diagonal of strained

aggregate. From Etchecopar A. 1977. A plane kinematic model

of progressive deformation in a polycrystalline aggregate.

Tectonophys. 39: 121–139. Reproduced with permission pend-

in a simple plastically deformed aggregate of grains

of a low-symmetry crystalline phase (Figure 17.19).

The array was homogeneously deformed by pure and

simple shear (Special Interest Box 17.1, Figures 17.20

and 17.21). In the model, the hexagons are allowed to

slip, rotate, move past neighbors, or even rupture, all

the while striving to minimize gaps or overlaps between

them. Results include the following points of interest:

1. Intergrain voids and overlaps are considerable, even

though the modeling procedure attempted to min-

imize them. In a real rock undergoing deformation,

these voids and overlaps must be compensated

for by bending, kinking, grain boundary sliding,

recrystallization, and pressure solution.

2. Slip planes nearly parallel to the plane of ﬂattening

in the array have low resolved shear stress and con-

sequently host grains strain less.

3. Increasing bulk strain of the aggregate results in a

stronger preferred orientation of slip planes and

therefore lattices as well as of grain shape. This has

an important bearing on the anisotropic fabric of

tectonites discussed later.